An embodiment of the present invention is a radiation emission device and more particularly, an X-ray tube. The embodiment can be used in medical imaging and also in the field of non-destructive controls when high-powered X-ray tubes are used. An embodiment of the invention is directed to cooling of such a device.
In radiology, X-rays are produced by an electron tube provided with an anode rotating on a shaft. A powerful electrical field created between the cathode and the anode enables electrons emitted by the cathode to strike the anode, generating X-rays. For this X-ray emission, the positive polarity is applied to the anode by the shaft, and the negative polarity is applied to the cathode. The unit is insulated especially by dielectric pieces or by an enclosure of the electron tube. This enclosure may be partly made of glass.
When the tube is used at high power, the impact of the electrons on the anode has the effect of abnormally heating this anode. If the power is excessively high, an emitter track of the anode may get deteriorated and pitted with impact holes. To prevent such overheating, the anode is made to rotate so that a constantly renewed and constantly cold surface is presented to the electron stream.
A motor of the tube therefore drives the shaft of the anode freely in a mechanical bearing. This shaft is located in an anode chamber. The anode chamber is itself formed in a support of the anode. On the one hand, the bearing is held by the anode support and, on the other hand, it holds the shaft of the anode.
In practice and when made on an industrial scale, this bearing comprises classic ball bearings as opposed to the little-used magnetic bearings. The problem posed by the rotating anodes arises from the fast wearing out of the metal coating the balls during the rotation of the shaft in the bearing. The service lifetime is then about 100 hours, giving a period of use of the tube of about six months to one year. To overcome this problem, it has been proposed to coat the bearings with metal, lead or silver in the form of a thin layer. To reduce this premature wearing out of the metal layer, a lubricant film is placed at the interface between the surfaces of the balls and the shaft, between the bearing and the shaft of the anode. To this end, the interior of the chamber is filled with a gallium-indium-tin based liquid. Such a liquid is chosen because it improves the coefficient of friction, reduces the noise of the impacts between the balls and augments heat transfer, due to the heating of the anode, to the fixed part, either by convection or by conduction. Other lubricant liquids are not chosen because they have poor degassing properties.
In current and future radiology the power needed by electron tubes is increasing in order to improve diagnosis. This increase in power is increasing the weight of the anode to six to eight kilograms. As a consequence, the resulting effects within the bearing are becoming critical. Furthermore, for use in computerized tomography with continuous rotation at two rotations per second, the bearing is subjected to acceleration of about eight G. Rotation speeds of three to four rotations per second are expected. Consequently, the service life of the bearing, and hence that of the tube, with the balls and the liquid, may be limited in time. Indeed, the liquid may lose its properties and therefore its qualities as and when heating and friction occur inside the bearing.
The use of a rotating anode must furthermore meet three main constraints. First, the rotation of the anode must be as free and as perfect as possible, and simple solutions of dynamic balancing must be planned to prevent the tube from vibrating when the anode rotates. Second, the anode must be capable of being taken to high voltage (normally, bearings with steel ball bearings serve this purpose). Third, the heat that is produced by the impact of the electrons on the anode target and propagates in the shaft must be efficiently discharged.
JP-A-5-258 691 describes an assembly in which ball bearings are lubricated by a gallium alloy. However, this assembly does not comply with the above constraints. Indeed, the balancing therein is difficult owing to the large diameter of the rotor, the thermal discharge is produced by a small-sized, fixed shaft, and there is nothing designed to improve the thermal and electrical conduction.
U.S. Pat. No. 6,125,168 describes for an X-ray tube, only the use of a gallium alloy to improve the thermal conduction. U.S. Pat. No. 6,160,868 also provides for improving the thermal conductivity with a gallium alloy. U.S. Pat. No. 6,377,658 is of the same type, and so is U.S. Pat. No. 6,192,107. U.S. Pat. No. 4,943,989 provides for the cooling of the anode itself. For thermal reasons, U.S. Pat. No. 3,719,847 provides for a liquid metal that evaporates and then returns to the liquid state. US 2003-0165217 provides only for a thermal shunt.
In any case, the cooling of the tube is a problem since it dictates the making of bigger tubes whereas, for reasons of handling, it is sought rather to make smaller tubes.